MONTREAL — “Canada is back,” says Justin Trudeau, the charismatic and bilingual prime minister of Canada, at international gatherings, seeking to showcase the imprint he wants to put on Canadian foreign policy in contrast to that of his predecessor, Stephen Harper. The prime minister has used very precise terms in his speeches: justice, environmental care, democracy and human rights. He even dared to invoke some of them during his official visit to China in September, although the Chinese did not applaud him for it.